In recent years, music distribution services for distributing music data via the Internet or the like have come to be widely used. In such music distribution services, encoded data obtained by encoding music signals are distributed as music data, but when the music signals are encoded, the bit rate of the music data is kept low to reduce a download time.
Generally, in the music data having a low bit rate, signals in a high frequency band which is difficult to hear with human ears are often cut to reduce data size. For this reason, even if the music data is decoded and regenerated, real impression given by original signals is lost, and sound is muffled. That is, high quality sound cannot be obtained.
Therefore, a technique has been provided which determines an extension start band according to side information of an input signal, divides the input signal into a plurality of sub-band signals, and extends a frequency band based on sub-band signals on a lower frequency side than the extension start band (e.g., see Patent Document 1). In this technique, a component of a high frequency band can be added to the input signal, and sound quality is improved.